This disclosure relates generally to electrical interconnection systems and more particularly to high speed electrical connectors.
Electrical connectors are used in many electronic systems. It is generally easier and more cost effective to manufacture a system on several printed circuit boards (“PCBs”) than to manufacture a system as a single assembly. Printed circuit boards are sometimes referred to as daughter boards or daughter cards, and are held in a card cage. Electrical connections are then established between the daughter cards.
A traditional arrangement for interconnecting daughter cards is to use a backplane. The backplane is a large PCB that contains signal traces that route electrical signals from one daughter card to another. The backplane is mounted at the back of the card cage assembly and the daughter cards are inserted from the front of the card cage. The daughter cards are parallel to each other and at right angles to the backplane.
For ease of assembly, the daughter cards are often connected to the backplane through a separable connector. Often, two-piece separable electrical connectors are used, where one connector is mounted to the daughter card, while another connector is mounted to the backplane. These connectors mate and establish numerous conducting paths. Sometimes, guide pins are attached to the backplane that guide the daughter card connector into proper alignment with the mating connector on the backplane.
Another traditional method for interconnecting daughter cards uses a midplane. In a midplane configuration, daughter cards are connected to both the front and the back of a large PCB, called the midplane. The midplane is typically mounted in the center of the card cage assembly, and daughter cards are inserted into both the front and the back of the card rack. The midplane is very similar to a backplane, but it has connectors on both sides to connect to daughter boards inserted from both the front and back of the assembly.
A further technique for interconnecting daughter cards is to directly connect orthogonal daughter cards without the use of a midplane. Electrical connectors are used to orthogonally interconnect the daughter cards, with each daughter card having a connector that mates with a connector of another daughter card.
The advantages of using a direct connect orthogonal configuration include flexibility of not being limited to a particular design of a midplane circuit board, better cooling due to absence of a midplane that can block airflow, and also reduced cost. However, using a direct connect orthogonal configuration also creates some challenges, including maintaining signal integrity when twisting internal signal conductors and ground conductors to interconnect two orthogonal daughter cards. Also a lack of a rigid physical support structure, such as a midplane or a backplane, that can provide mechanical alignment for the daughter cards can create challenges.
One of the difficulties in making a high density, high speed connector is that electrical conductors in the connector can be so close that there can be electrical interference between adjacent signal conductors. To reduce interference, and to otherwise provide desirable electrical properties, shield members may be placed between or around adjacent signal conductors. The shields are typically grounded conductors that prevent signals carried on one signal conductor from creating “crosstalk” on another signal conductor. The ground conductors also impact the impedance of each signal conductor, which can further contribute to desirable electrical properties.
Other techniques may be used to control the performance of a connector. Transmitting signals differentially can also reduce crosstalk. Differential signals are carried on a pair of conducting paths, called a “differential pair.” The voltage difference between the conductive paths represents the signal. In general, a differential pair is designed with preferential coupling between the conducting paths of the pair. For example, the two conducting paths of a differential pair may be arranged to run closer to each other than to adjacent signal paths in the connector. Shielding in the form of ground conductors may be used between differential pairs.
Maintaining signal integrity can be a particular challenge in a direct connect orthogonal configuration. It is often desirable to have a uniform impedance throughout the path of a signal conductor, as abrupt changes in impedance may alter the signal integrity. However, the impedance of conductive elements, such as signal conductors and/or ground conductors, may be altered in the vicinity of changes in spacing between signal and ground conductors or other changes along the signal path. Such changes are difficult to avoid in a direct connect orthogonal connector in which the signal conductors need to be routed from a board to another orthogonal board.
Furthermore, at the mating interface, force must be generated to press conductive elements from the separable connectors together so that a reliable electrical connection is made between the two conductive elements. Frequently, this force is generated by spring characteristics of the mating portions in one of the connectors. For example, the mating portions of one connector may contain one or more members shaped as beams. As the connectors are pressed together, each beam is deflected by a mating contact, shaped as a post, pin or blade in the other connector. The spring force generated by the beam as it is deflected provides a contact force.
The need to generate mechanical force imposes requirements on the shape of the mating portions. For example, the mating portions must be large enough to generate sufficient force to make a reliable electrical connection. These mechanical requirements may preclude the use of shielding, or may dictate the use of conductive material in places that alters the impedance of the conductive elements in the vicinity of the mating interface. Because abrupt changes in impedance may alter the signal integrity of a signal conductor, mating portions are often accepted as being noisier portions of a connector.